Heretofore, colored vulcanized elastomers have often been prepared by mixing finely divided pigments with uncured rubber followed by vulcanization of the rubber composition. Such elastomers generally have a very uniform color which is resistant to abrasion in the sense that a pigment is dispersed throughout the rubber composition.
U.S. Pat. No. 3,577,261 provides a quick method for identifying uncured rubber stocks and rubber cements by the effect of a dye therein on an adherent layer applied thereto. The adherent layer includes particulate pigments and a solvent. The solvent picks up dye from the substrate of rubber stock or rubber cement and imparts the color of the dye to the adherent layer. It is implied that the color of the dye within the rubber stock or rubber cement is hard to visually perceive because of colored pigments (such as carbon black) in the substrate.
U.S. Pat. No. 1,895,088 issued January of 1933 and relates to colored stainable rubber products and a method of coloring. The reference teaches volatile solvent-based dyeing of translucent rubbers and color stabilization by a transparent varnish. The reference teaches opaque rubbers mask the dye color and thus should not be used. The reference teaches that staining ingredients migrate in the rubber unless barrier layers are present to inhibit the migration.
White sidewalls and raised white letters on modern pneumatic tires are often composed of stain resistant elastomers, particularly elastomers having large amounts of isobutylene repeating units such as butyl rubber, halobutyl rubber, as well as EPDMs. These butyl rubbers are used so that common contaminants on the roadways (such as motor oils, salt, cinders, and tars) and staining oils and vulcanization accelerators within the adjoining black elastomer portions of the tire do not readily stain or discolor the white regions. These white sidewall compositions are so stain resistant that they do not require barrier layers such as taught in U.S. Pat. No. 1,895,088. White sidewalls of modern pneumatic tires also contain a large percentage of titanium dioxide that has a refractive index much higher than elastomers so that the final white sidewall composition in the tire is opaque. Since a black sidewall is always beneath the white sidewall, high opacity is desired to maximize apparent whiteness.